## 37.4. Stop Syntax Errors in String Tests

Using the test or [ (square bracket) command (Section 35.26) for a string test can cause errors if the variable starts with a dash (`-`). For example:

>     if [ "$var" = something ]
>     then ...

If `$var` starts with `-r`, the test command may think that you want to test for a readable file.

One common fix (that doesn't always work; see below) is to put an extra character at the start of each side of the test. This means the first argument will never start with a dash; it won't look like an option:

>     if [ "X$var" = Xsomething ]
>     then ...

That trick doesn't work if you want the test to fail when the variable is empty or not set. Here's a Bourne shell test that handles empty variables:

>     case "${var+X}" in
>     X) ...do this if variable is set...
>        ;;

>     *) ...do this if variable is not set...
>        ;;
>     esac

If `$var` is set (even if it has an empty string), the shell replaces ${var+X} (Section 36.7) with just `X` and the first part of the case succeeds. Otherwise the default case, `*)`, is used.

See also Section 37.3 for a brief example of bash parameter expansion and dealing with unset or null values by reporting an error or by assigning default values.

-- JP

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